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The scope that has been used for
imaging so far has been the 6" reflector. The 10" is a bit too big for the
EQ6 mount, but a larger mount is on order. To prepare for it, certain
modifications to the 10" have been made.
Flocking The inside of the tube had been
painted with matt-black paint. Dark though this might be, stray light
reflecting from sources such as the Moon or planets such as Jupiter or Saturn,
could bounce around inside the tube and end up in the eyepiece. At worst this
would cause visible flaring, and at best, a loss of contrast. A solution is to coat the inside of the tube with 'flocking
material', which consists of many short black fibres. In theory, the fibres
are synthetic and do not shed and end up on the mirror. The flocking was
obtained from
Telescope
Parts UK.
The
picture shows the tube after one sheet has been applied.The difference between
the light reflected by the flocking and the black paint is
startling.
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The flocking
already has an adhesive attaching it to a plastic backing sheet. Removing the
whole sheet from the backing would end up with an unmanageable sticky problem,
so I decided to leave the backing sheet in place and glue that to the scope
interior. Easier said than done! Any slight distortion causes ripples and bubbles, which are very
difficult to remove. The tube is so long that I can only just reach inside to
the centre. Contact adhesive bonds instantly and there is then no easy way to
correct mistakes. However, once the plastic
backing is glued securely, the corners of the flocking can be peeled back and
then cuts can be made in the plastic backing to make the creases lie down. The
flocking can then be carefully lowered back onto the backing sheet.
Imperfections are purely cosmetic, but I still spent many hours trying to
produce a flat flocked surface. |
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Two sheets in place... |
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Three
sheets... |
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And
finally completed. Not a pleasant job to do
because it is awkward and prone to disaster! |
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Secondary mirror supports
On the few pictures taken
with the 10" it was seen that the diffraction spikes around bright stars were
split.
This is caused by the vanes holding
the secondary mirror not being aligned correctly. Ideally each spike should be
at 90 degrees to its neighbours. Visual inspection of
the vanes showed that this was far from the case.
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The secondary
was detached and the vanes removed from the tube. The vanes were straightened,
and the screw holes in the tube were repositioned. Angles were checked to be as close to 90 degrees as possible, and
the vanes reinserted. Care was taken to get the centre of the secondary holder
as close to the centre of the tube as possible. Maximum error as measured with
digital vernier calipers was+- 1.6mm.
This is a photograph of the adjusted
vanes in situ.
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Niels
Noordhoek's Maskulator
software was used to simulate the diffraction pattern produced by the above
picture.
The spikes
should not be split!
It will be interesting to see what
actual images look like, and a comparison with the simulation can be
made.
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28th October |
Clear
sky at last to try out the flocking, the new focus mount, and improved
collimation.
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30 second frame of
Altair
Encouraging result!
The spikes are at 90 degrees to each other and not
split. The image was unguided and stars are quite
small.
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The equivalent taken with the
6 inch reflector (3 vanes) - also 30 second exposure.
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The
success of the flocking in the 10" reflector prompted me to renovate the small
scope. This is after applying the flocking.
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The focuser needs to be aligned with
the centre of the tube. This was done by wrapping paper around the inside of
the tube with a line exactly at the half way point. The laser collimator dot
shows up neatly on the line. |
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The 6" mirror after washing and
replacing in the plywood cell. The mirror was made and aluminised in 1985 and
apart from a few small blemishes, it is still looking quite good. |
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